1. Field of the Invention
The present invention relates to a thin steel sheet excellent in ridging resistance and formability. More specifically, the invention is concerned with developed results in developments and researches on the basis of the experimental acknowledgement that manufacturing steps can be reduced with no cold rolling step involved by controlling rolling conditions.
2. Related Art Statement
High ductility and high lankford value (r-value) are required in the case of thin steel sheets of a thickness of about 2 mm or less which are used as construction material, automobile vehicle body material, canning material and various surface-treated raw plates so as to attain excellent bending formability, bulging formability, and drawability.
The number of parts to be bulged in the forming process has been recently increasing to improve the yield of the steel sheet in the forming. Because, the bulging can reduce inflow of a material from wrinkle holding portion in the forming. Particularly, a high n-value (not less than 0.23) (strain hardening exponent) is required as the material characteristics for this purpose.
Even if the formability is excellent in a particular direction but planar anisotropy is large, wrinkles are formed after the forming, because the actual forming is two dimensional. If the anisotropy is small, an amount of an ear to be cut is small to reduce a blank area, thereby enhancing the yield of the steel sheet to a large extent. Such a mechanical anisotropy can be evaluated by .DELTA.El (anisotropic parameter of elongation) and .DELTA.r (anisotropic parameter of r value). .DELTA.El.ltoreq.5% and .DELTA.r.ltoreq.0.5 are required for the steel sheets excellently low in the anisotropy.
Further, the steel sheet to be formed is basically required to be excellent in the strength-elongation balance. This is because the steel sheet poor in the strength-elongation balance causes troubles such as wall cracks during the forming.
Particularly, when the high strenghtening is aimed at to reduce the thickness of the steel sheet, the strength-elongation balance becomes an important characteristic.
In this case, the realization of the following relation is an approximate indication showing that the steel sheet is excellent in the strength-elongation balance: EQU T.S. (kg/mm.sup.2).times.El(%).gtoreq.1,500
Since these materials are mainly used on the outermost side of the finally formed products, the surface properties after the forming have come to be important.
Further, the steel sheets for automobiles are required to undergo a pretreatment before coating, that is, phosphate coating. For this reason, the phosphate coating property becomes one of factors as the properties of the steel sheets. If the phosphate coating property is not good, the succeeding bake-on coating is not successful.
Moreover, the demand for the corrosion resistance of the formable thin steel sheets has recently become severer and severer, and the use of the surface treated steel sheets have rapidly increased.
Since the automobiles used in North Europe and North America are required to withstand the corrosion by snow-melting salt agent, they are required to have severer corrosion resistance.
On the other hand, even if the surface treated steel sheet is specially used, the corrosion resistance will be deteriorated under the conditions that the steel sheets are likely to be damaged during forming. Thus, the adhesion between the steel sheet as the base and the surface treated layer is extremely important for the surface treated steel sheet.
Furthermore, steel sheets for automobiles are required to be thinner to improve the fuel consumption of the automobiles. There occurs a problem in thus thinned steel sheet that the bulging rigidity of the formed product is lowered. For this reason, the formed product is easily deflected when an external force is applied thereto. On the other hand, since the bulging rigidity of the steel sheet is proportional to the Young's modulus, to increase the Young's modulus of the steel sheet plane is to increase the bulging property of the steel sheet. In this case, excellent bulging property can be obtained if the average Young's modulus among those in three directions, i.e., a rolling direction (hereinafter referred to as L direction), a direction orthogonal to the rolling direction (hereinafter referred to as C direction) and a direction extending at 45.degree. with respect to the rolling direction (hereinafter referred to as D direction) is not less than 22,000 kg/mm.sup.2.
These formable thin steel sheets are ordinarily produced in the following steps:
Mainly, a low carbon steel is first used as a raw steel material, and converted into a steel slab of a thickness of about 200 mm by continuous casting or ingot making-slabbing, which is converted into a hot rolled steel sheet of a thickness of about 3 mm through hot rolling. This hot rolled steel sheet is subsequently pickled and cold rolled to obtain a steel sheet of a desired thickness, which is subjected to a recrystallization treatment through box annealing or continuous annealing to obtain a final product.
The largest defect of this manufacturing process is that the steps are lengthy, and energy, number of staff and time necessary for obtaining the product are not only huge but also various problems on the quality, particularly the surface properties, of the product disadvantageously take place during the long manufacturing steps.
As mentioned above, it has been indispensable to included the cold rolling step (rolling temperature: less than 300.degree. C.) in the process of producing the formable thin steel sheets.
The cold rolling step not only attains the desired reduction of thickness, but also serves to promote the growth of crystalline grains in the orientation of (111), which is advantageous for the deep drawability, in the final annealing step through utilization of the plastic strain introduced by the cold forming.
However, since the deformation resistance of the steel sheet is extremely higher in the cold forming as compared with the hot forming, energy required for rolling is huge and wear of the rolling rolls is considerable. In addition, rolling troubles such as slip are likely to occur.
To the contrary, if rolling is possible and particularly excellent formability is obtained at a relatively higher temperature range (so-called warm temperature range) of not less than 300.degree. C. to not more than 800.degree. C., the above problems can be completely removed to give large merits in the production.
On the other hand, there is a large problem in the production through the warm rolling. This is ridging. The ridging is a defect of the surface unevenness produced during the forming of the product. Thus, since the formed product is used mainly on the outermost side of the articles, this is a fatal defect for this steel sheet.
Metallogically speaking, the ridging is originated from the fact that a group of crystal oriented grains (for instance, a group of [100]-oriented grains) difficult to be divided even after undergoing formingrecrystallization step remains as being expanded in a rolling direction. In general, the ridging is likely to occur in a circumstance in which forming is carried out at a relatively high temperature in a ferrite (.alpha.) range as in a warm rolling. Particularly, when the draft in the warm range is high (that is, as in the case of the production of the thin steel sheet), the ridging is conspicuous.
With complication and high grade tendency of the formed products, these formable steel sheets frequently undergo severe forming, and therefore are required to have excellent ridging resistance.
By the way, processes of producing iron and steel materials have recently remarkably varied, and the formable thin steel sheets are not exceptional, either.
That is, according to the conventional processes, a molten steel is converted to a steel slab of a thickness of about 250 mm through ingot making-slabbing, which is uniformly heated and soaked in a heating furnace and converted into a sheet bar of a thickness of about 30 mm in a rough hot rolling step, and then converted into a hot rolled steel strip of a desired thickness through finish hot rolling. To the contrary, recently, the slabbing step has first been able to be omitted through introduction of the continuous casting process and there is a tendency that the heating temperature of the steel slab is reduced from a conventional temperature of around 1,200.degree. C. to around 1,100.degree. C. or a lower temperature aiming at the improvement of the material characteristics and energy saving.
On the other hand, there has been being practically used a new process in which a steel sheet of a thickness of not more than 50 mm is instantly produced from a molten steel to omit the heating treatment and the roughly rolling step in the hot rolling.
However, these new producing processes are disadvantageous in that they all fracture the tissues (cast tissues) formed through solidification of the molten steel. Particularly, it is extremely difficult to break the strong cast texture having {100} &lt;uvw&gt; as main orientation formed during the solidification.
As a result, ridging is likely to occur in the final steel sheet, and particularly the warm rolling promotes the ridging.
There have been heretofore disclosed some processes for producing the deep drawable steel sheets by warm rolling, for instance, in Japanese Patent Publication No. 47-30,809, and Japanese Patent Application Laid-open Nos. 49-86,214, 59-93,835, 59-133,325, 59-85,729 and 59-226,149. They are all characterized in that recrystallization treatment is carried out immediately after rolling in a warm range, and are an innovative technique which enables omission of the cold rolling step.
However, these prior art techniques have paid no attention to the improvement on the above ridging resistance. In this respect, the warm rolling is generally less advantageous than the cold rolling with respect to the ridging resistance of the thin steel sheet.